Remove colons in merge message, since a colon between the 'fixes' keyword and the issue number will not be interpreted by github as closing an issue
Merge pull request #8 from twisted/fix_contributing_doc

Remove colons in merge message example in contributing doc.

Reviewers @tomprince
Fixes #5

If there is a colon between 'fixes' and the issue number, Github fails to interpret this as a directive to close the issue.  
Fix the example merge message in the contributing doc to not have colons, and further call this out in the contributing doc so contributors notice the lack of colon.